This invention relates to a process of producing composite powder which consists of metal sulfide particles coated with copper in a thin film. More particularly, the invention is concerned with a process of producing such powder by coating the particles of a metal sulfide, such as molybdenum disulfide or tungsten disulfide, with copper in the form of a thin layer made by a cementation reaction between copper ions and a metal and/or an alloy baser than copper.
The copper-coated metal sulfide powder thus obtained is exceedingly desirable as solid lubricant for use in the self-lubricating powder materials to be shaped and sintered to oilless bearings, sliding parts, and the like.
The self-lubricating parts such as those of oilless bearings have usually been prepared by mixing the powder of copper, tin, or other metal, as the base, with the powder of molybdenum disulfide, graphite or other solid lubricant, pressing the mixture in dies to produce compressed shapes, sintering the shapes, and finally impregnating the products with oil. However, even if such solid lubricating powder of molybdenum disulfide, graphite or the like, is directly added to the base metal powder, a good self-lubricating product is not obtainable for a number of reasons including the infeasibility of achieving uniform mixing and low strength attained on sintering. Therefore, it is desirable that the powder of solid lubricant, as of molybdenum disulfide or graphite, be not employed directly as it is but in the form of a composite powder in which the individual particles are thinly coated with copper, nickel, or other second metal.
Such composite powders are available in a number of known and possible combinations of solid lubricant particles as cores and metals in the form of coating films. Typical of them are the combinations of molybdenum disulfide, tungsten disulfide or other metal sulfide powder and a copper coating.
Methods of producing the composite powder have heretofore depended on thermal decomposition, reduction, vapor-phase reaction, plating reaction, and vacuum evaporation. However, they have a variety of disadvantages in the process of manufacture, quality of the product, and in respect of the equipment required. No method has been established yet which will produce a composite powder of a high, stable quality in a simplified process on an industrial scale. More recently, some proposals have been made for the manufacture of composite powder. One of them is a method, disclosed by Japanese Patent Application Public Disclosure No. 32436/1976, for coating core particles of a non-metal, alloy, or metal with copper. The method consists of dispersing the core particles in an ammoniacal ammonium salt solution containing dissolved nickel, reacting the resulting slurry with a reducing gas at a high temperature and a high pressure so that the core particles can be coated with nickel, taking out the nickel-coated powder from the solution, redispersing the particles in a solution of copper adjusted to pH 7 or less, and obtaining copper-coated powder by replacing nickel with copper through a cementation reaction. According to the method, it is not until the nickel once applied on the core particles has been replaced by copper that the eventual copper coating is attained. The double procedure necessitates an accordingly large number of steps, calling for much time and labor.
Another example is Japanese Patent Application Public Disclosure No. 82871/1976. It teaches a technique aimed at coating the particles of a powdery lubricant, e.g., graphite, molybdenum disulfide, or boron nitride, with one of various metals, e.g., copper, silver, nickel, iron, or aluminum. In this method, the powder of a lubricant ranging in particle size from 1 to 2000.mu. is mixed into a dispersion of aforementioned metal powder, in the range of 0.05-500.mu., in a solvent, thereby allowing the metal particles to adhere to the lubricant particles, and after the filtering, in order to attain more strong and stable adhesion, the coated particles obtained are baked with heat in a hydrogen stream or a mixed stream of hydrogen and nitrogen. This method again requires much time and labor because of the very cumbersome steps of mixing, separation by filtration, and heating following the filtration. Apparently the adhesion of the metal particles upon the lubricant particles is weak and they have to be handled with the greatest possible care prior to the heating.
From the foregoing it is not too much to say that the two methods described above admit of much improvement, especially in simplification of the steps. The present invention has been arrived at after an extensive search for a method of manufacturing composite powder on an industrial scale in a more simplified and yet more positive way than the prior art techniques, the method being primarily directed to the typical composite powder consisting of metal-sulfide type solid lubricant core particles and copper coating.
It has now been found that the coating of metal sulfide particles with copper can be done in a simple way of mixing and succeeding cementation treatment in a single vessel, by suitably choosing the particle size of the metal sulfide, composition of the copper solution, kind and size of the metal to be added, state of agitation and conditions for cementation. The present method needs no step of preliminarily applying nickel on the lubricant particles, transferring the powder to another vessel, or heating the coated particles. Experiments have indicated that the composite powder thus prepared, when added as a solid lubricating part, would give an excellent self-lubricating powder material to be sintered. Thus, the method of the invention is quite satisfactory not only because of the simplicity but also from the viewpoint of quality of the product.